Common three-phase industrial motors are currently being operated with variable frequency drive (VFD) systems to provide more efficient starting capability and energy consumption. In VFD applications, the usually large starting currents are avoided; torque per ampere values are high at any given frequency along the speed-torque curve.
One observed drawback of VFD-driven motors, however, is the large voltage drops across the stator at reduced frequencies. Below an operating frequency of 15 Hz, this effect is considerable. VFD-driven motors sustain increased thermal stresses and higher peak voltages. The inverters of the VFD switching systems usually produce peak voltages that are 1.5 to 2 times as great as the nominal voltage of a nominal voltage supply (compared to 1.4 times as great as the voltage of a sinusoidal power supply). Such large voltage peaks that recur over the cycle of the power waveform create stresses on the insulation of the stator winding.
Insulation degradation also results from extremely fast voltage rise times, which cause an unevenly distributed voltage drop in the motor stator. Eighty percent of the peak voltage appears across the first turn of the first coil of the winding. The ground insulation separating the stator coils from the frame may also be potentially harmed.
Also apparent is a damped "ringing" waveform that appears at the front and rear of each pulse, which thus contributes to transient voltage amplitudes.
Common practice has been to increase the thickness and better the quality of the insulation material used in the stators of VFD-driven motors. While an increased thickness and an improved quality result in higher costs, neither option offers a solution toward protecting the first turn of the coil, which has an insulation that cannot effectively be increased.
A further complication of the aforementioned problem is that, during manufacture, the stator coil is wound randomly. Therefore, during fabrication a statistical variation in the insulation thickness is created. Another unpredictability in manufacture results in pinhole-sized imperfections, producing an uncertainty as to the viability of the insulation.
Lately, it has been suggested that motor failures could be reduced more effectively by eliminating or greatly reducing peak voltages and voltage rise times. Such suggested changes, however, have been directed at modifying the drive system, rather than improving the insulation of the stator wiring.
The present invention reflects the discovery that the motor or coil lead wire insulation can be specially fabricated to eliminate or greatly reduce the peak voltages and transient voltage peaks traveling along the stator wires.
In accordance with the current invention, a new type of "filter line" cable has been perfected that will reduce internal voltage interferences traveling down the wires of the VFD motors. To attenuate harmful voltage peaks, this new type of "filter line" uses insulation containing ferrite.
As will be demonstrated hereinafter in the examples presented in the preferred embodiment, voltages at the coil are dampened considerably by the insulation developed by this invention. The band width of the inventive cables is considerably narrower than standard cables used as motor lead wire. Traditionally, motor and coils were left exposed to gentle sign wave power. The introduction of invertors in the motor design, however, has generated pulsating DC with fast rise times. This has caused the motors to experience additional electrical stresses.
This invention is not to be confused, however, with the insulation containing ferrite referred to as "filter line", as illustrated in U.S. Pat. Nos. 5,170,010 (issued to Mahmoud Aldissi on Dec. 8, 1992) and 5,171,937 (also issued to Mahmoud Aldissi on Dec. 15, 1992). These already patented types of "filter line" cable are ones that are primarily shielded from external interferences (such as EMI and RFI) that penetrate the cable layers radially. The present invention distinctly seeks to attenuate voltage transients and peak currents traveling along the wire.